Melt Filter for Purifying Plastic Melts

ABSTRACT

The invention relates to a melt filter ( 1 ) for purifying, in particular, plastic melts leaving extruders, comprising a filter disk ( 3 ), which is provided between two plates ( 5, 5 ′) that form a housing equipped with a filter element changing station ( 7 ), and which can be rotationally driven by means of a power-operated pawl-type drive ( 4 ). Said filter disk has recesses, which are separated by webs ( 11 ) along a circular path and which serve to accommodate filter elements ( 8 ) that, by means of perforated disks, are supported against the forces arising due to the drop in pressure occurring therein in the flowing direction of the plastic melt. The inventive melt filter also comprises a melt channel ( 9 ), which passes through the plates in the area of the circular path, feeds the melt to the filter elements, and which widens up to the filter elements. The aim of the invention is to design the smallest possible melt filter with which, during operation, few unwanted changes occur in the plastic, and which ensures, even in the event of high pressures, a filter element exchange with nearly a constant pressure while remaining cost-effective. To this end, the invention provides that the plates completely cover the filter disk, whereby at least one of the plates is interrupted by the filter element changing station, and that the filter changing station is designed such that it is larger than a filter element and smaller than or the same size of two filter elements.

The invention concerns a melt filter for purifying especially plastic melts discharged by extruders, with a filter disk, which is installed between two plates that form a housing equipped with a filter element changing station, can be rotationally driven by a power-driven ratchet drive, and has recesses that are separated by webs and arranged along a circular path for holding exchangeable filter elements, which are supported by means of perforated disks against the forces that arise due to the pressure drop that occurs in them in the direction of flow of the plastic melt, and with a melt channel, which passes through the plates in the area of the circular path, feeds the melt to the filter elements, and widens towards the filter elements.

Prior-art melt filters are described, for example, in EP 0 114 651 B1. However, the previously known melt filter has a very large and thus expensive filter disk, whose filter element changing station is also very large, but the surface of the filter disk against which the melt flows is very small, so that extremely poor efficiency results. EP 0 569 866 A1 has already proposed that the melt channel be widened towards the filter elements, so that the melt to be filtered can flow against two filter elements at the same time, but here again, there is no flow against large areas of the filter elements, so that similarly poor efficiency is obtained.

In addition, the teeth on the filter disk have a relatively large pitch, so that when the filter disk is rotated further, large surfaces of the dirty filter elements are exchanged for correspondingly large surfaces of clean filter elements. This results in pressure differences in the cleaned melt which are unacceptable during further processing of the melt, so that additional pumps are often needed to guarantee a constant pressure of the cleaned melt.

It has already been proposed that gear drives that produce smaller steps be used instead of the sturdy, inexpensive ratchet drive in order to exchange only small filter disk areas with dirty filter elements for filter disk areas with clean filters at any given time and thus to ensure pressure constancy. However, expensive rotational drives of this type constitute an immense cost factor.

DE 42 12 928 A1 has already disclosed a large-surface cover for a filter disk, but large areas of the filter disk are still exposed to ambient air when the filter disk is rotated, so that undesired changes in the plastic can occur.

The objective of the invention is to specify a melt filter that is as small as possible, with which hardly any changes occur in the plastic during operation, which guarantees filter element exchange at more or less constant pressure, even at high pressures, and which is nevertheless inexpensive to produce.

To this end, it is proposed that the plates completely cover the filter disk, with at least one of the plates being interrupted by the filter element changing station, and that the filter element changing station be designed larger than one filter element and smaller than or the same size as two filter elements. This results in the formation of a housing which encloses, if possible, the whole filter disk and is better able to withstand the high pressures that are required.

It has been found to be effective for at least one of the plates to have a reversibly movable region that covers the filter element changing station, for the filter disk to be completely covered during the operation of the filter and closed snugly towards the filter disk, and for the filter element changing station to be uncovered for the filter element change to be carried out during the operation of the filter. On the one hand, the complete covering of the filter disk makes it possible to realize higher pressures, and, on the other hand, it is guaranteed that during the operation no plastic melt adhering to the filter disk comes into contact with the ambient air.

It is advantageous that the given distance between filter elements against which the melt is flowing and the filter element changing station is larger than or the same size as the width of one filter element and a web, and smaller than the width of two filter elements and a web. This guarantees that melt flows against the largest possible surface area of the filter disk with the smallest possible filter changing station without it being possible for melt to be pressed out of the filter element changing station.

Due to these optimum relationships between the size of the filter element changing station and the surface of the filter disk against which melt is flowing, the filter disk can be made more compact than the prior-art filter disks and yet make a larger effective filter surface available.

It is advantageous if the ratio of the web area against which the melt is flowing to the area of the filter disk through which melt is flowing is less than 18% and greater than 12%. This guarantees that the webs are provided with dimensions that still enable them to withstand the high pressures but oppose the melt to be filtered with the least possible surface areas that cannot be used for filtration, so that the greatest possible filter surface area can be effectively realized. In this connection, it has been found to be effective if the ratio of the web area against which the melt flows, to the area through which the melt flows, is 15±1%.

To be able to guarantee constant pressure during the filter change, it is advantageous that, for each stroke of the ratchet drive, a maximum of 10% of the area of the filter disk against which the plastic melt flows can be exchanged for corresponding filter disk areas with unused filter elements. In this connection, it has been found to be effective if 6±1% of the filter area can be exchanged per stroke of the ratchet drive. The exchange of a maximum of 10% of the area of the filter disk, i.e., of the filter elements and the webs, against which the plastic melt flows guarantees that approximately constant pressure is present in the filtered melt, and this allows trouble-free further processing of the melt in the downstream machines.

The invention will now be explained in greater detail with reference to the drawing, which shows a melt filter 1 that consists of a housing 2, a filter disk 3, and a ratchet drive 4. The housing 2 is formed by a plate 5, which is connected by fastening devices 6 with another plate 5′, whose outline is indicated by a broken line. The plates 5, 5′ enclose the filter disk 3 between them. A filter element changing station 7 is indicated in the plate 5. It is essentially the same size as a filter element 8. In addition, the plate 5 has a melt channel 9, which widens towards the filter disk 3 in the form of an annular segment 10.

In addition to the filter elements 8, the filter disk 3 has webs 11. Teeth 12, which interact with the ratchet drive 4, are arranged along the periphery of the filter disk. The webs 11 are connected by a wheel rim 13.

Due to the fact that the filter element changing station 7 is selected to be as small as possible, most of the filter disk 3 can be enclosed by the plates 5. This makes it possible to handle the largest possible pressure prevailing in the melt channel without the occurrence of clogging of the filter disk 3 in the housing 2. The webs 11 and the peripheral wheel rim 13 of the filter disk 3 are supported on the plates 5, 5′ and seal the melt channel 9 and the annular segment 10 towards the outside.

The annular segment 10 spans the webs 11′ to 11′″″ and the filter elements 8′ to 8″″″. In this regard, as a result of the relationships, in accordance with the invention, between the size of the surface against which the melt is flowing, the distance to the filter element changing station, and the size of the filter element changing station, an optimum condition is achieved, so that even with a small filter disk 3 at high pressures, the melt can be optimally filtered with good pressure constancy.

LIST OF REFERENCE NUMBERS

-   1. melt filter -   2. housing -   3. filter disk -   4. ratchet drive -   5. plate -   6. fastening device -   7. filter element changing station -   8. filter element -   9. melt channel -   10. annular segment -   11. webs -   12. teeth -   13. wheel rim 

1. Melt filter (1) for purifying especially plastic melts discharged by extruders, with a filter disk (3), which is installed between two plates (5, 5′) that form a housing [2] equipped with a filter element changing station (7), can be rotationally driven by a power-driven ratchet drive (4), and has recesses that are separated by webs (11) and arranged along a circular path for holding exchangeable filter elements (8), which are supported by means of perforated disks against the forces that arise due to the pressure drop that occurs in them in the direction of flow of the plastic melt, and with a melt channel (9), which passes through the plates (5, 5′) in the area of the circular path, feeds the melt to the filter elements (8), and widens towards the filter elements (8), wherein the plates (5, 5′) completely cover the filter disk (3), with at least one of the plates (5, 5′) being interrupted by the filter element changing station (7), and that the filter element changing station (7) is designed to be larger than one filter element (8) and smaller than or the same size as two filter elements (8, 8′).
 2. Melt filter in accordance with claim 1, wherein at least one of the plates (5, 5′) has a reversibly movable region that covers the filter element changing station (7), that the filter disk (3) is completely covered during the operation of the filter and is closed snugly towards the filter disk, and that the filter element changing station (7) is uncovered for the filter element change.
 3. Melt filter in accordance with claim 1, wherein the given distance between filter elements (8) against which the melt is flowing and the filter element changing station (7) is larger than or the same size as the width of one filter element (8) and a web (11) and smaller than the width of two filter elements (8, 8′) and a web (11).
 4. Melt filter in accordance with claim 3, wherein the ratio of the web area against which the melt is flowing to the area of the filter disk (3) through which melt is flowing is less than 18% and greater than 12%.
 5. Melt filter in accordance with claim 4, wherein the ratio of the web area against which the melt is flowing to the area of the filter disk (3) through which melt is flowing is 14% to 16%.
 6. Melt filter in accordance with claim 3, wherein, for each stroke of the ratchet drive (4), a maximum of 10% of the area of the filter disk (3) against which the plastic melt flows can be exchanged for a corresponding filter disk area with unused filter elements (8).
 7. Melt filter in accordance with claim 6, wherein, for each stroke of the ratchet drive (4), 5% to 7% of the area of the filter disk (3) against which the plastic melt flows can be exchanged for a corresponding filter disk area with unused filter elements (8). 